Color improvement treatment of dialkylamides



United States Patent ()fifice 3,316,298 COLOR IMPROVEMENT TREATMENT OFDIALKYLAMIDES William K. Wilkinson, Waynesboro, Va., assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Filed Dec. 11, 1963, Ser. No.329,869

6 Claims. (Cl. 260-561) This invention relates to a process forimproving the color of dialkylamides. More particularly, it relates tothe processing of impure dialkylamides for use as solvents in spinningoperations.

In the spinning of spandex fibers and acrylonitrile fibers, it is commonpractice to use the lower dialkylamides of lower fatty acids, such asN,N-dimethylformamide and ,N-dimethyl-acetamide, as polymer solvents.These materials are good solvents for such polymers, and solutionsthereof may be extruded or dry spun into filaments of high quality.Because of economic considerations, it is desirable to recover theevaporated solvent and reuse it. In practice, most of the solvent isrecovered from the spinning cells by condensation from the aspirationgases, but since it has been subjected to relatively high temperaturesduring the spinning operation, it is contaminated with impurities.

Known purification processes for the dialkylamides involve distillationor deionization or both. It has been observed on occasion that suchprocesses do not completely remove all impurities from the recoveredsolvent. In particular, the solvent retains either colored impurities orimpurities which generate color on exposure to ultraviolet light. It isbelieved that these impurities include nitrosoamines which are formedduring the spinning operation. As used herein, the term UV color refersto the color in the solvent after exposure to sunlight or intenseultraviolet light.

An object of this invention is provision of dialkylamide solventssubstantially free of color and color-forming bodies and useful in theformation of polymer solutions and resulting yarns of improved color andquality. Another object of the invention is the purification ofrecovered dimethylacetamide solvent for use in spinning spandex fibers.

These and other objects are accomplished by a process of purifying alower dialkylamide of a lower fatty acid which comprises treating thedialkylamide with hydrogen peroxide and distilling off impurities whichboil at temperatures below that of said dialkyamide. In a preferredembodiment of this process, the pH of the dialkylamide is adjusted to atleast 9.

The solvents which may be purified by the process of this invention arethe lower dialkyl-amides of the lower fatty acids. These includedimethylformamide, diethylformamide, dimethylacetamide,dimethylpropionamide, and diethylacetamide. The essential step in thepractice of this invention is the treatment of the recovereddialkylamide with hydrogen peroxide. The temperature and duration of theperoxide treatment is generally not critical, but ordinarily it shouldbe for at least ten minutes at 155 C. The amount of hydrogen peroxideused should be in the range of from about 0.1% to about by weight, basedon the dialkyamide solvent being treated. Large amounts of peroxide arenot necessary for the practice of this invention, and preferably theamount used is in the range from 0.1% to 0.5%, by weight. Withdimethylacetamide, best results are obtained when using from 0.2% to0.3% of hydrogen peroxide. Any convenient method of adding the peroxidemay be used. The use of the commercially available 35% hydrogen peroxidein sufiicient amounts to yield 3,3 16,298 Patented Apr. 25, 1967 theconcentrations set forth above produces excellent results.

In the practice of this invention, it is necessary, for optimum results,to adjust the pH of the dialkylamide solvent to a value in the rangefrom about 9 to about 11. Since the hydrogen peroxide is essentiallyneutral, the pH adjustment may be made before or after the hydrogenperoxide is added. If the pH of the dialkylamide is too low, it isbrought into the proper range by the addition of a suitable basicmaterial. Suitable materials include ammonium hydroxide and the lowerdialkylamines. The preferred material is diethylamine which, in ordinarycases, is required in amounts ranging from about 0.1% to about 0.6%, byweight, based on the dialkylamide.

After treatment with the hydrogen peroxide at the specified pH, thetreated dialkylamide is then stripped of lowboiling impurities bydistillation. These impurities comprise, mainly, water and thedialkylamines, including dimethylamine, diethylamine, etc. In thedistillation step,

it is found to be beneficial if a substantial rate of purging thedistillation column accompanies the distillation process. A purge rateof from about 10% to about 25% of the feed is found to be particularlyhelpful.

Although not essential for the practice of this invention, a heattreatment prior to the distillation step described above mayadvantageously be applied to the peroxide-treated dialkylamide. Thus,heating the peroxide-treated solvent to a temperature of about -160 C.for |l015 minutes before distilling off the low-boiling impurities isparticularly beneficial.

Further purification procedures known in the art may be carried out onthe product from this invention, if desired. Thus, the residue from theabove-described distillation may be subjected to a second distillation.In this subsequent distillation, the product is obtained as adistillate, preferably at a substantial reflux ratio, such as in therange from about 5:1 to about 6:1. Moreover, conventional deionizationprocedures may be applied to the product, if desired, whereby thesolvent is passed through beds of cationic resins and beds of anionicresins, or alternatively, through a bed of mixed resins for furtherpurification in accordance with the art.

The best method contemplated for carrying out this invention is asfollows: the pH of condensed dimethylacetamide is adjusted to the rangeof 9-11 with diethylamine and is then mixed with sufficient aqueous 35%hydrogen peroxide to give a mixture containing from 0.10.5% peroxide.This mixture is fed continuously to a heater at 155160 C. In the heater,the colorforming impurities are oxidized, and the organic peroxideswhich are formed are partially decomposed before being fed to adistillation column. The efiluent from the heater is fed to a firstcolumn from which the lowboiling impurities are distilled off. Theproduct is taken off at the base of the column and fed to a seconddistillation column from which it is distilled as a vapor, condensed,and passed through a column containing cationic and anionic resins toprovide continuous deionization.

This invention is further illustrated, but is not intended to belimited, by the following examples in which parts and percentages are byweight unless otherwise specified. The color of the products isdetermined by electrophotometric measurement (using light of 425millimicrons wave length) and is reported herein as Hazen color numbers,which are the same as the widely used scale of the American PublicHealth Association. On this scale, distilled water is zero, and standardsamples of stable inorganic salt solutions provide recognizedcalibration points. UV color is reported in Hazen numbers after exposureof a The aspiration condensate containing about 2% water is obtainedfrom a cell in which a spandex fiber is dry spun from a solution indimethylacetamide. The pH of this condensate is adjusted to a level of10 with diethylamine. Sufiicient 35% hydrogen peroxide is then added togive a mixture containing 0.25% hydrogen peroxide. This mixture isheated for about 15 minutes to a temperature of 155160 C. and is thenfed to a distillation column from which water and other low-boilingimpurities are distilled off. The dehydrated product is taken off at thebase of the column as a vapor, condensed, and fed to a seconddistillation column from which it is distilled. The distillate is passedthrough a bed consisting of a mixture of Amberlite resins IR-200 and'IRA-400. The product has a color (Hazen number) of 0.3 and a UV color of10.2.

Similar condensate, similarly processed except for deletion of thetreatment with hydrogen peroxide and diethylamine, has a color of and aUV color" of 42.

Example 11 Dimethylacetamide aspiration condensate, recovered from aspandex dry spinning process, is mixed with sufficient 35% hydrogenperoxide solution to give a mixture containing 2% hydrogen peroxide. Themixture is heated under reflux for approximately 25 minutes. Water andlow-boiling impurities are then distilled off and discarded as a firstfraction. A second fraction is distilled and collected at the boilingpoint of dimethylacetamide. This product has a color of 1 1 and a UVcolor" of 14.

Similar condensate, processed in the same manner except for deletion ofthe treatment with hydrogen peroxide, has a color of 9 and a UV color of192.

The process of this invention has the advantage over other purificationprocedures in that the decomposition products of hydrogen peroxide arewater and oxygen which are easily removed from the treated dialkylamideson distillation. Moreover, under the conditions specified herein, themixture of hydrogen peroxide and dialkylamide may be safely handled in alarge-scale operation without danger of build-up of hazardousconcentrations of organic peroxides. In addition, use of hydrogenTrademai-k for the Rohm and Haas Companys ion exchange resins.

peroxide, particularly at low concentrations, avoids fouling of thecalandria or heating elements of the distillation columns by theoxidizing agent or its decomposition products.

It is not known with certainty what the color-forming impurities inrecovered dialkylami-de are, but it is believed that they are convertedby the hydrogen peroxide to materials which are easily removed ondistillation. Dialkylamides recovered from a spandex spinning processand purified by the process of this invention are reusable in thepreparation of spandex spinning solutions free of objectionable color.

As many widely diiferent embodiments of this invention may be madewithout departing from the spirit and scope thereof, it is to beunderstood that this invention is not to be limited to the specificembodiments thereof except as defined in the appended claims.

I claim:

1. A process for treating a lower dialkylamide of a lower fatty acid toremove color-producing impurities which comprises adding from about 0.1%to about 5% by weight of hydrogen peroxide to said dialkylamide,distilling the hydrogen peroxide-containing mixture at a temperaturebelow the boiling point of said dialkylamide, and thereafter collectingthe dialkylami-de.

2. The process of claim 1 wherein the pH of the dialkylamide is adjustedto a value from about 9 to about 11 before the mixture is distilled.

3. The process of claim 2 wherein the mixture is heated for a period offrom about ten to fifteen minutes at a temperature from about C. to C.before the distillation step.

4. The process of claim 3 wherein said dialkylamide isdimethylacetamide.

5. The process of claim 3 wherein said mixture is fed to a distillationcolumn using a purge rate of from about 10% to about 25%.

6. The process of claim 1 wherein said dialkylamide is the solventrecovered from a spinning operation.

References Cited by the Examiner UNITED STATES PATENTS 2,767,204 10/1956Thompson 260-452 2,928,860 3/1960 Harrington 260-458 OTHER REFERENCESSchumb et al.: Hydrogen Peroxide, pages 574-5, New York, Reinhold, 1955.

Wallace: Hydrogen Peroxide in Organic Chemistry, pages 46 and 59, DuPont, 1960.

WALTER A. MODANCE, Primary Examiner. NATALIE TROUSOF, AssistantExaminer.

1. A PROCESS FOR TREATING A LOWER DEALKYLAMIDE OF A LOWER FATTY ACID TOREMOVE COLOR-PRODUCING IMPURITIES WHICH COMPRISES ADDING FROM ABOUT 0.1%TO ABOUT 5% BY WEIGHT OF HYDROGEN PROVIDE TO SAID DIALKYLAMIDE,DISTILLING THE HYDROGEN PEROXIDE-CONTAINING MIXTURE AT A TEMPERATUREBELOW THE BOILING POINT OF SAID DIALKYLAMIDE, AND THEREAFTER COLLECTINGTHE DIALKYLAMIDE.